Can I use non existing CSS classes?

I have a table where I show/hide a full column by jQuery via a CSS class that doesn't exist:

<table>
   <thead>
      <tr>
         <th></th>
         <th class="target"></th>
         <th></th>
      </tr>
   </thead>
   <tbody>
      <tr>
         <td></td>
         <td class="target"></td>
         <td></td>
      </tr>
      <tr>
         <td></td>
         <td class="target"></td>
         <td></td>
      </tr>
   </tbody>
</table>

With this DOM I can do this in one line via jQuery: $('.target').css('display','none');

This works perfectly, but is it valid to use CSS classes that aren't defined? Should I create an empty class for it?

<style>.target{}</style>

Are there any side effects or is there a better way to do this?

"CSS class" is a misnomer; class is an attribute (or a property, in terms of scripting) that you assign to HTML elements. In other words, you declare classes in HTML, not CSS, so in your case the "target" class does in fact exist on those specific elements, and your markup is perfectly valid as it is.

This doesn't necessarily mean that you need to have a class declared in the HTML before you can use it in CSS either. See ruakh's comment. Whether or not a selector is valid depends entirely on the selector syntax, and CSS has its own set of rules for handling parsing errors, none of which concern the markup at all. Essentially, this means HTML and CSS are completely independent of each other in the validity aspect.¹

Once you understand that, it becomes clear that there is no side effect of not defining a .target rule in your stylesheet.² When you assign classes to your elements, you can reference those elements by those classes either in a stylesheet, or a script, or both. Neither has a dependency on the other. Instead, they both refer to the markup (or, more precisely, its DOM representation). This principle applies even if you're using JavaScript to apply styles, as you're doing in your jQuery one-liner.

When you write a CSS rule with a class selector, all you're saying is "I want to apply styles to elements that belong to this class." Similarly, when you write a script to retrieve elements by a certain class name, you're saying "I want to do things with elements that belong to this class." Whether or not there are elements that belong to the class in question is a separate issue altogether.

---

¹ _{This is also why a CSS ID selector matches all elements with the given ID regardless of whether the ID appears exactly once, or multiple times (resulting in a non-conforming HTML document).}

² _{The only situation I'm aware of where an empty CSS rule like that is necessary is when some browsers refuse to apply certain other rules properly as the result of a bug; creating an empty rule will cause those other rules to be applied for some reason. See this answer for an example of such a bug. However this is on the CSS side and therefore should have nothing to do with the markup.}

There are no ill effects to use classes which don't have styles. Indeed, that's part of the usefulness of CSS is that it's de-coupled from the markup and can style or not style elements/classes/etc. as needed.

Don't think of them as "CSS classes." Think of them as "classes" which CSS happens to also use if it needs to.

According to HTML5 specification:

> A class attribute must have a value that is a set of space-separated > tokens representing the various classes that the element belongs to. > ... There are no additional restrictions on the tokens authors can use in > the class attribute, but authors are encouraged to use values that > describe the nature of the content, rather than values that describe > the desired presentation of the content.

Also, in the version 4:

> The class attribute has several roles in HTML: > > - As a style sheet selector (when an author wishes to assign style > information to a set of elements). > - For general purpose processing by > user agents.

Your use case falls under the second scenario, which makes it a legitimate example of using a class attribute.

You can use a class which has no styles, this is entirely valid HTML.

A class referenced in a CSS file is not a definition of a class, it is used as a selector rule for styling purposes.

When you use a classname in JavaScript, it does not look at the CSS to find that class. It looks directly in the HTML code.

All that is required is that the classname is in the HTML. It does not need to be in the CSS.

In fact, many people think it's actually a good idea to keep separate classes use with CSS and Javascript, as it allows your designers and coders to work independently without getting in each other's way by using each other's classes.

(note, the above paragraph is obviously more applicable for larger projects, so don't feel that you have to go to this extreme if you're working on your own; I mentioned it to make the point that the two can be entirely separate)

You can use CSS classes without using it, but I suggest that if you are adding CSS classes just for the JavaScript/jQuery code, prefix with it js-YourClassName so the front-end developers never use these classes to style the elements. They should understand that these classes can be removed at any time.

The moment you add the Class in your HTML the Class will be defined, so your solution is completely fine

It's not necessary to define CSS classes in your stylesheet. It should work just fine. However, adding it won't harm.

One thing that nobody here has fully mentioned is that JavaScript (aided by jQuery in this case) isn't able to directly modify a document's cascading style sheet. jQuery's css() method merely changes the matched set of elements' style property. CSS and JavaScript are completely unrelated in this aspect.

$('.target').css('display','none'); doesn't change your .target { } CSS declaration at all. What has happened here instead is that any element with a class of "target" now looks something like this:

<element class="target" style="display:none;"></element>

Are there any side effects caused by not pre-defining a CSS style rule? None whatsoever.

Is there a better way to do this? Performance-wise, yes there is!

How can the performance be improved?

Rather than directly modifying the style of each element, instead you can pre-define a new class and add that to your matched elements using addClass() (another jQuery method).

Based on this pre-existing JSPerf which compares css() with addClass(), we can see that addClass() is actually much faster:

How can we implement this ourselves?

Firstly we can add in our new CSS declaration:

.hidden {
    display: none;
}

Your HTML would remain the same, this pre-defined class is simply in place for later use.

We can now modify the JavaScript to use addClass() instead:

$('.target').addClass('hidden');

When running this code, rather than directly modifying the style property of each of your matched "target" elements, this new class will now have been added:

<element class="target hidden"></element>

With the new "hidden" class, this element will inherit the styling declared in your CSS and your element will be set to no longer display.

As is mentioned by so many others, yes, using classes with no assigned CSS is perfectly valid and rather than thinking of them as 'CSS classes' you should simply recognise the semantics of class and ID to be groups and individual elements respectively.

I wanted to chip in as I felt an important point hasn't been raised given the example. If you ever need to do visual manipulations to a variable length of elements (in this case you're using table rows) then it always makes sense to recognise that the cost of doing so through Javascript could potentially be very expensive (e.g if you have thousands of rows).

In this situation let's say we know that column 2 always has the potential to be hidden (it's a conscious function of your table) then it makes sense to design a CSS style to handle this use case.

table.target-hidden .target { display: none; }

Then rather than using JS to traverse through the DOM finding N elements we simply need to toggle a class on one (our table).

$("table").addClass("target-hidden")

By assigning the table an ID this would be even quicker and you could even just refer to the column by using the :nth-child selector which would reduce your markup further but I can't comment on efficiency. Another reason for doing this is that I hate inline styling, and will go to great lengths to eradicate it!

It will have no effect if you apply a class on a HTML element, and that class is not defined in CSS. It is a common practice and like Aamir afridi said if you are using classes for js only purpose, it is a good practice to prefix them with js- .

It is not only valid for calsses, but also for id attribute of html elements.

There's no problem at all of using classes to just query for elements. I used to give such class names the sys- prefix (for example, I'll name your class sys-target) to distinguish them from classes used for styling. This was a convention used by some microsoft developers in the past. I also noticed a growing practice of using the js- prefix for this purpose.

If you are not comfortable with using classes for purposes other than styling, I recommend using the Role.js jQuery plugin which allows you to achieve the same purpose using the role attribute, so, you may write your markup as <td role="target"> and query for it using $("@target"). The project page has good description and examples. I use this plugin for big projects because I really like keeping classes for styling purposes only.

Refer to the jQuery validation engine. Even in there we also use non-existent classes to add validation rules on the HTML attributes. So there is nothing wrong in using classes that are not actually declared in a stylesheet.